This invention relates to a high performance spindle assembly for forced controlled polishing of metals, plastics, or other materials requiring the application of a constant force. More particularly, the present invention relates to a spindle assembly for force controlled polishing or planarization of semiconductor wafers.
Systems for applying force to a workpiece, such as a semiconductor wafer, have in the past measured the pressure applied at the spindle or focused on monitoring the position of the spindle relative to the polishing surface. For example, one version of a spindle assembly for use in polishing a workpiece includes a vertically oriented, spring counter-balanced spindle utilizing a single-acting diaphragm cylinder as a down force mechanism. A pneumatic proportional regulator is used with a built-in pressure transducer feedback loop connected to the diaphragm cylinder in order to control force output of the diaphragm cylinder.
In operation, this system lowers the spindle to a correct polishing height using two different cylinder mechanisms. First, a conventional piston cylinder lowers the spindle to a mechanical hard stop. Then, by pressurizing a diaphragm cylinder, the spindle is moved until the workpiece reaches the polishing surface. The pressure transducer in this device monitors the performance of the diaphragm cylinder and feeds back the result to the pneumatic proportional regulator. Because the pressure transducer in this system is an integral component of the E/P regulator supplying pressure to the diaphragm cylinder, the pneumatic proportional regulator does not receive the actual force placed on the workpiece by the spindle. In practice, a certain amount of the pressure applied by the diaphragm piston is lost due to friction losses in the cylinder, bearings, and various other mechanical components of the spindle assembly. This friction is not seen by the pneumatic proportional regulator because the differential pressure transducer only receives information from the diaphragm cylinder.
Another system for providing a continuous down force to a workpiece being polished includes a rolling diaphragm cylinder operatively connected to a spindle through a linkage system. The spindle is guided by a spline bearing that allows longitudinal and rotational movement by the spindle. The diaphragm cylinder is controlled by a servo valve as part of a proportional amplifier loop that receives feedback from between the servo valve and diaphragm cylinder to monitor position/force applied to a workpiece. Again, the friction due to the diaphragm cylinder, and the additional friction added by the spline bearing guiding the spindle, are not seen in the control loop of the servo valve. Due to the friction, a hysteresis effect may be experienced at the output. This effect reduces the accuracy of position/force measurements. Accordingly, a spindle drive assembly is necessary that provides improved downforce accuracy.